After reading up on the CommentAPI, if I were to write the RatingAPI, I'd pretty much plagiarize the CommentAPI but only make the following changes:
Remove the title and link elements (they are unnecessary).
Change the html discovery to:
<link rel="service.rating" type="text/xml" href="url goes here" title="Rating Interface" min="0" max="5"/>
Change the new comment element to a rating element:
What do you think?